Flash memory device and program method thereof

ABSTRACT

A flash memory device includes a memory cell array on which data is stored, and page buffers that are connected to the memory cells through the bit lines and apply one of the first voltage, second voltage or third voltage between the first and second voltage, to the respective bit line when performing the program.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 11/856,699, filed on Sep. 17, 2007, which claims priority from Korean Patent Application No. 2006-96007, filed on Sep. 29, 2006 and Korean Patent Application No. 2007-63576, filed on Jun. 27, 2007, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a flash memory device a program method thereof, and more particularly, to a flash memory device and program method capable of increasing a programming speed.

Generally, a flash memory device includes an insulating layer, a floating gate, a dielectric layer and a control gate, which are laminated on a semiconductor substrate. The floating gate is used as a charge storing layer, and the details thereof will be described.

When a program voltage is applied to a control gate through a selected word line, a Fowler-Nordheim (F-N) tunneling phenomenon is produced between a semiconductor substrate and a floating gate and thus electrons flow into the floating gate from the semiconductor substrate to perform a program operation.

A floating gate storing electrons becomes a programmed cell to be higher in a voltage than a threshold voltage of an erase, and thus the programmed cell can be distinguished from the erase cell by reading this distribution difference of a threshold voltage.

A flash memory device has two states, for example, an erase state or a programmed state, and the memory device driven in one erase state and one programmed state refers to as a single level chip (“SLC”). In the other hands, a programming method of each memory cell in multi-level to store much more data than the single line chip SLC has been developed, which is referred to as a multi level chip (“MLC”). The multi line chip MLC is operated in a way of defining each data state which is different each other in respective distribution intervals of the threshold voltage. More details thereof will be described.

For example, when data is stored on the multi line chip MLC, the states that one memory cell may have can be sorted into an erase state, a PV1 state, a PV2 state and PV3 state. Here, assuming that the PV1 state is the programmed state with the lowest interval among the programmed threshold voltage intervals, the PV2 state is higher in threshold distributions than the PV1 state, and the PV3 state is higher than the PV2 state. Like this, with respect to the respective threshold voltage intervals, data of multi bit (i.e., 11, 01, 00 and 01) may be defined in sequence. In the following, a description thereof will be made with reference to FIG. 1.

FIGS. 1A to 1D show a conventional program method of a flash memory device. Each threshold voltage interval that a memory cell may have sorted into an erase state, a PV1 state, a PV2 state and PV3 state, and data value of 2 bit (multi bit) is defined to each state. A lower bit of data values of 2 bit refers to as a low page, and a higher bit refers to as a high page. A program operation of the low page refers to as a low page program “LSB program”, and a program operation of the high page refers to as a high page program “MSB program”.

The programming sequence of the multi line chip MLC will be described.

Firstly, in memory cell array configured by a unit of block, all of the memory cells of the selected blocks are erased (FIG. 1A). Subsequently, a low page program LSB operation is performed.

In the low page program LSB operation, among memory cells of erase states, a ground voltage is applied to the bit lines of the selected memory cells and a source voltage is applied to the bit lines of the non-selected memory cells. The low page program LSB operation is performed in such a way that a program voltage is applied to the selected word lines and a pass voltage is applied to the non-selected word lines. Accordingly, through these procedures of the low page program LSB operation, the selected memory cells of erase states become PV1 state (FIG. 1B).

The high page program MSB operations may be sorted into a first high page program MSB and a second high page program MSB.

The first high page program MSB operation is an operation for programming the selected memory cells into the PV2 state. To program the erased memory cells into the PV2 state, the selected memory cells are LSB-programmed to the PV1 state from the erase state and then the first MSB program is performed thereto to make the selected memory cells be in PV2 state (FIG. 1C).

The second high page program MSB operation is an operation for programming the selected memory cells into the PV3 state (FIG. 1D). The second high page program MSB is performed in such a way that among memory cells of erase states, a ground voltage is applied to the bit lines of the selected memory cells and a program voltage is applied to the word lines connected to the selected memory cells.

A program operation, speed of the multi level chip MLC may be reduced since each program operation has to be performed to program cells from erase states to the respective programmed state (PV1 state, PV2 state or PV3 state).

BRIEF SUMMARY OF THE INVENTION

According to the present invention, in a program operation of a multi level chip having various intervals of program threshold voltages, different voltages between the threshold voltages are applied to the bit lines and thus the program operation having different threshold voltage intervals can be performed at the same time. Accordingly, the operation frequency of program can be decreased to reduce program operation time.

A program method of a flash memory device according to one embodiment of the present invention, comprises performing a first program for programming cells to a first state and a second state higher than the first state, and performing a second program simultaneously together with the first program, for programming cells to the second state and a third state higher than the second state.

The first program is performed for the selected cells among the cells of erase states to be the first state.

The selected cells are the cells which will be programmed to the first state and second state, and when performing the first program, the cells which will be programmed to the second state is programmed simultaneously to the first state together with the cells which will be programmed to the first state.

The second program is performed for programming the selected cells among the cells of erase states to the third state, and at the same time programming the cells which will be programmed to the second state among the first programmed cells, to the second state.

When performing the second program, a positive voltage is applied to the bit lines connected to the cells to be programmed to the second state.

The positive voltage of difference between a threshold voltage of the second state and a threshold voltage of the third state is applied.

A method of programming a flash memory device that has a erase state, a first state, a second state and a third state, according to another embodiment of the present invention, the method comprises performing a first program such that some of the first memory cells of erase states become to the second memory cell of the first state, and performing a second program in such a way that a ground voltage is applied to the first bit lines connected to the strings including the first memory cells, a positive voltage is applied to the second bit line connected to the strings including the second memory cells so that the first memory cells become the third memory cells of the second state and the second memory cells become the fourth memory cells of the third state.

In the second state, the threshold voltage becomes higher than that of the first state by the second program operation, and in the third state, the threshold voltage becomes higher than that of the second state by the second program operation.

The positive voltage is the voltage that is higher than the ground voltage, and is lower than a difference of threshold voltage between the to program voltage applied to the selected bit line and the drain select transistor included in the string.

A method of programming a flash memory device according to another embodiment of the present invention comprises performing a first program for programming some of the erased cells to the first state and the second state higher than the first state, and performing a second program in such a way that a ground voltage is applied to the first bit line connected to the string including the selected memory cells for programming some of erased cells to the third state higher than the second state, and at the same time a positive voltage is applied to the second bit line connected to memory cells for programming cells that is first-programmed, to the second state.

The positive voltage of difference between the threshold voltages of the second state and third state is applied.

A flash memory device according to the present invention comprises a memory cell array on which data is stored, and page buffers that is connected to the memory cells through the bit lines and applies one of the first voltage, second voltage or third voltage between the first and second voltage, to the respective bit line when performing the program.

The first voltage is a source voltage and the second voltage is a ground voltage and the third voltage is a positive voltage.

The positive voltage of a difference between the threshold voltages of the second state and third state is applied.

The positive voltage is transmitted in such way that a turn voltage of the element transmitting voltage from the page buffer to the bit lines is not turned wholly but slightly.

An embodiment of the present invention will be explained with reference to the accompanying drawings. However, the invention is not limited to the disclosed embodiment. Additional advantages, objects, and features of the invention will be set forth in the description which follows and will become apparent to those having ordinary skill in the art upon examination of the following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D illustrate a conventional method of programming a flash memory, respectively.

FIG. 2 is a circuit diagram showing a program method of a flash memory device according to the present invention.

FIG. 3 is a circuit diagram showing a page buffer of FIG. 2.

FIG. 4 is a circuit diagram showing a part of FIG. 2 to explain a program method of a flash memory device according to the present invention.

FIGS. 5A to 5F show a program method of a flash memory device according to the present invention in sequence.

FIG. 6 is a graph comparing program frequencies of a flash memory device between the present invention and the prior art.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 2 is a circuit diagram showing a program method of a flash memory device according to the present invention.

With reference to FIG. 2, one cell block 100 constituting a memory cell array and page buffers connected thereto are shown. The cell block 100 includes a plurality of strings S₀ to S_(k). Each string is configured in such a way that a drain select transistor DST, memory cells F₀ to F_(n) and a source select transistor SST are connected in series. Each drain select transistor DST is connected to each of bit lines BL₀ to BL_(K) to transmit voltages of the page buffer PB to the strings S₀ to S_(k) or receive voltages of the page buffer PB from the strings S₀ to S_(k). The respective source select transistor SST is connected to a common source line CSL. A plurality of memory cells F₀ to F_(n) are arranged in series between the drain select transistor DST and the source select transistor SST.

A gate electrode of the drain select transistor DST shares the drain select lines DSL. Each gate electrode of the source select transistor SST shares the source select line SSL and is connected thereto.

The page buffer PB transmits voltages corresponding to the operations of program and erase to bit lines BL₀ to BL_(K) when programming and erasing, and receives voltage applied from the bit lines BL₀ to BL_(K) when reading.

In the present invention, in addition to a first voltage (i.e. 0V) and a second voltage (i.e., Vcc) that is produced in a typical program operation, a third voltage Vd between the first voltage and the second voltage is further produced. The third voltage Vd is a positive voltage between the first voltage and the second voltage, and it will be described with reference to FIG. 3.

FIG. 3 is a circuit diagram showing an operation of the page buffer of FIG. 2.

With reference to FIG. 3, even the page buffer PB used in the present invention is the page buffer that produces further the third voltage as described above, the configuration thereof is similar to that of a conventional page buffer PB. However, one of the first to third voltages can be transmitted selectively from the page buffer PB to the bit line BL by adjusting turn on voltage of some elements among the elements constituting the page buffer PB. For convenience of understanding, the page buffer PB will be described briefly referring to the FIG. 3.

The page buffer PB includes a select circuit 32 for selecting a plurality of bit lines and is configured in a dual latch design.

In more specific description, a precharge element P1 is implanted by a PMOS transistor that is operated in response to a precharge signal PRECHb, and is connected between a source voltage Vcc and a sensing node S0. A program element P2 is operated in response to a program signal PGM and is connected between the sensing nod S0 and a first node E1. A first control element P3 and a second control element P4 are connected in series between a second node E2 and a ground Vss, and the first control element P3 is operated in response to a voltage of the sensing node S0 and the second control element P4 is operated in response to a first latch signal LAT1. A reset element P5 is operated in response to a first reset signal RST1 and is connected between the first node E1 and the ground Vss. A first latch 33 is connected between the first node E1 and the second node F2 and comprises two inverters I3, I4. A transmitting element P6 is operated in response to a transmitting signal PDUMP and is connected between the sensing node S₀ and the third node E3. A third control element P7 and a fourth control element P8 are connected in series between the third node E3 and the ground Vss, and the third control element P7 is operated in response to the voltage applied to the sensing node S₀ and the fourth control element P8 is operated in response to the second latch signal LAT2. A second reset element P9 is operated in response to a second reset element RST2 and is connected between a fourth node E4 and the ground Vss. A second latch 34 is connected between the third node E3 and the fourth node E4, includes two inverters I5, I6. The data inputted to the second latch 34 is stored thereon according to the operations of a first input element P10 and a second input element P11. The first input element P10 is operated in response to a first input signal DI and is connected between the fourth node E4 and a fifth node E5. The second input element P11 is operated in response to a second input signal nDI and is connected between the third node E3 and a fifth node E5. An input and output element P12 is operated in response to a input and output signal PBDO and is connected between the first node E1 and the fifth node E5, and the fifth node E5 is connected to a input and output line DI0.

The select circuit 32 includes an even charge element P13, an odd charge element P14, an even select element P15 and an odd select element P16 for connecting the bit lines BLe, BLo to the page buffer PB. The even charge element P13 and the odd charge element P14 is connected in series between bit lines BLe, BLo through the sixth node E6. The even charge element P13 is operated in response to an even charge signal DISCHe and in connected between the even bit line BLe and the sixth node E6, the odd charge element P14 is operated in response to an odd charge signal DISCHe and is connected between the odd bit line BLo and sixth node E6. A charge voltage VIRPWR is applied to the sixth node E. The even select element P15 is operated in response to a even select signal BSLe and is connected between the sensing node S0 and the even bit line BLe. The odd select element P16 is operated in response to an odd select signal BSLo and is connected between the sensing node So and the odd bit line BLo.

While operating program, if the program signal PGM is activated, the program element P2 is turned on and thus the voltage of the first latch 33 is transmitted to the sending node So. The voltage transmitted to the sensing node So is transmitted to the even bit line BLe or the odd bit line BLo through the even select element P15 or the odd select element P16 of the select circuit 32.

In the other hands, when a third voltage is transmitted to the even or odd bit line BLe or BLo, if the program element P2 is turned on not wholly but slightly and the select signal selected BSLe or BSLo is activated, the third voltage, which is positive voltage between the first voltage and second voltage, can be outputted to the bit line. Additionally, if the program signal PGM is activated and the select signal selected BSLe or BSLo is turned on slightly, the third voltage can be outputted to the bit line. At this time, it is preferable that a program operation is performed in a state of the voltage of bit line BLe or BLo being OV.

Subsequently, the program method according to the present invention will be described in detail referring to one page 110 of FIG. 2.

FIG. 4 is a circuit diagram showing a part of FIG. 2 to explain program method of a flash memory device according to the present invention. According to one aspect of the present invention, among the memory cells 0F₀ to kF₀ in the selected page, it is programmed such that a 0^(th) cell 0F₀ is an erase state, a first cell 1F₀ is the PV1 state, a second cell 2F₀ is the PV2 state, a third cell 3F₀ is the PV3 state and a k^(th) cell kF₀ is the PV3 state. At this time, the erase state, PV1 state, PV2 state and PV3 state refer to as 11, 10, 00 and 01, respectively. However, this can be changed depending on definitions thereof, and in the present invention, depending on the order of increasing the threshold voltage, the program states are defined as a erase state, a PV1 state, a PV2 state and a PV3 state. The examples thereof will be described with reference to FIGS. 5A to 5F.

FIGS. 5A to 5F show a program method of a flash memory device according to the present invention in sequence.

Referring to FIG. 5A, before performing a program operation, an erase operation is performed by a unit of cell block so that memory cells (all memory cell of cell block including memory cell 0F₀ to kF₀) become a first memory cell of an erase state. The erase operation is performed such that the first voltage (i.e., 0V) is applied to a selected word line WLO and the second voltage (i.e., Vcc) is connected to the all bit lines BLo to BLk. According to the erase operation, a threshold voltage distribution becomes an erase state (FIG. 5B).

Referring to FIG. 5C, a first program operation as a low page program LSB operation is performed for programming the first cell (1F₀) to be the second memory cell of PV1 state. At this time, the low page program LSB operation is performed simultaneously to the second and K^(th) cells 2F₀ and KF₀, which is to be programmed for being PV2 state, to become PV1 state.

In more details, while the low page programming LSB, a program voltage is applied to the selected word line WLO, and the first voltage (i.e., 0V) is applied to the selected bit lines BL1, BL2 and BLk and the second voltage (i.e., Vcc) is applied to the non-selected bit lines BL0 and BL3. As a result, the 0^(th) cell (0F₀) keeps the erase state, and the first cell (1F₀), second cell (2F₀) and K^(th) cell (kF₀) become the PV1 state (FIG. 5D).

Referring to FIG. 5E, a second program operation as a high page program MSB operation is performed to program the third cell (3F₀) to become the PV3 state. At the same time, the second cell 2F₀ and the K^(th) cell KF₀ are programmed to be the PV2 state. More detailed description thereof is as follows.

A program voltage is applied to the selected word line WLO and the first voltage (i.e., 0V) is applied to the bit line BL3 connected to the third cell 3F₀ for programming the third cell 3F₀ so as to be the PV3 state. At the same time, the third voltage Vd is applied to the bit line BL2 connected to the second cell 2F₀ for programming the second cell to be the PV2 state having the threshold voltage distribution higher than the PV1 state and lower than the PV2 state (FIG. 5F).

At this time, if a verify voltage of the PV1 state refers to Va, a verify voltage of the PV2 state refers to Vb and a verify voltage of the PV3 state refers to Vc, it is preferable that the third voltage Vd as much as voltage difference of Vc and Vb is applied, since the threshold voltage of the PV2 state has to be distributed between the threshold voltages of the PV1 state and PV3 state.

There may be various factors determining the threshold voltage distributions, however, it is mainly dependent on the amount of electrons stored on a floating gate. The amount of electrons stored on a floating gate can be determined by a voltage difference between word lines and a semiconductor substrate.

If the program voltage is applied to word lines and a ground voltage is applied to a channel of a semiconductor substrate through bit lines, a coupling phenomenon between a control gate and a floating gate by the program voltage is produced. This coupling phenomenon induces a tunneling phenomenon through which electrons flow to the floating gate from the semiconductor substrate, and a threshold voltage of the programmed cell can be varied dependent on the amount of tunneled electrons. Accordingly, an important factor determining the threshold voltage of the programmed cell is a voltage difference between the program voltage applied to the word line and the voltage applied to the bit line. With respect to this, a detailed description will be given as follows referring to a graph.

FIG. 6 is a graph comparing program frequencies of a flash memory device between the present invention and the prior art.

With reference to FIG. 6, x axe indicates a program frequency and y axe indicates a threshold voltage. In the prior art, for programming to be the PV3 state, firstly a program operation (a) is performed to reach the PV2 state, and then a program operation (b) has to be performed further to raise the threshold voltage to the PV3 state.

However, in the present invention, the voltage Vd as same as difference of threshold voltages of the PV2 sate and PV3 state is applied to the bit lines simultaneously that are connected to the cell which will be programmed to become the PV2 state and thus through only one program operation (A) the program of the PV2 state and PV3 state can be performed to reduce a program operation time.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

In the present invention, when performing a program operation of a multi level chip having a plurality of threshold voltage intervals, different voltages as same as difference of threshold voltages are applied to bit lines and thus a program operation of cell having different threshold voltage intervals can be performed at the same time. Accordingly, the frequency of a plurality of program operation can be decreased to reduce program operation time. 

1. A flash memory device comprising: a memory cell array having a plurality of strings each of which is coupled to a bit line; and page buffers each of which is coupled to the strings through the each bit line and has at least a latch, configured to apply a first voltage, a second voltage lower than the first voltage, and a third voltage between the first and the second voltages to the each bit line.
 2. A flash memory device according to claim 1, wherein the first voltage is a source voltage and the second voltage is a ground voltage and the third voltage is a positive voltage between the source voltage and the ground voltage.
 3. A flash memory device according to claim 2, wherein the third voltage is applied to the bit line by controlling a turn on voltage of a transistor that is connected between the bit line and the latch.
 4. A flash memory device according to claim 2, wherein the third voltage is applied to the bit line by lowering the turn on voltage of the transistor.
 5. A flash memory device according to claim 3, wherein the positive voltage is transmitted in such way that the transistor is not turned on wholly but slightly.
 6. A flash memory device comprising: a memory string having a plurality of memory cells, wherein the memory string is coupled to a bit line; and a page buffer configured to apply one of a first voltage, a second voltage, and a third voltage to the bit line, wherein the first voltage is applied to the bit line when a selected memory cell is not programmed, the second voltage is applied to the bit line when the selected memory cell is programmed to a target level, and the third voltage is applied to the bit line when the selected memory cell is programmed to a level lower than the target level.
 7. A flash memory device according to claim 6, wherein the first voltage is a source voltage, the second voltage is a ground voltage, and the third voltage is a positive voltage between the source voltage and the ground voltage. 